In the field of packaging and more specifically of packaging in net bags, strips of plastic material are used for making the bags which are applied to one side or to the two sides of a net tube and cooperate in order to close the net tube and sometimes contribute to the bags having the required mechanical features.
These strips are obtained by transversally cutting a continuous strip which, in turn, is obtained by longitudinally cutting an initial band. To this end, it is common to unroll the band from a band supply and longitudinally cut it by means of a cutting die which is repeatedly applied against longitudinally contiguous sections of the band. The cutting die can have various cutting edges which, in pairs, determine both strips to be individualized of the initial band.
In the advanced preparation of bags, the strips intended to be used in a bag have a specific design or pattern such that the cutting edge of the die has a particular configuration for giving the strip a shape which follows this specific pattern.
When the die is a rotary die, it is possible to use a die-carrying drum, the exterior circumference of which is adjusted to the length or to a multiple of the length of this pattern. However, this requires the die-carrying drum and the associated die having to be replaced each time that the length of the pattern needs to be changed.
In order to avoid this drawback, replacement cutting dies can be used. These cutting dies can be in the shape of metallic plates which are applied, for example using magnetic attraction forces separately on the exterior surface of a die-carrying drum. Each cutting die will have cutting edges, the length of which will depend on the length of the pattern which is desired to be obtained, which will not necessarily be equal to the perimeter which the die carrier provides. Consequently, it may occur that in the replacement dies, the cutting edges do not turn around the die-carrier completely and that the cutting edge is thus interrupted, there being at least one cutting start point and one cutting end point. The systems which use replacement dies, in which this situation occurs, are enabled for stopping the advance of the band in a cutting operation of the same during the rotation of the die carrier and during the times when there is an interruption to the cutting edge, that is when there is no interference between the cutting edge and the band.
Notwithstanding the foregoing, even though the cutting start and end points of the same cutting edge are aligned on the plate, i.e. they have the same ordinate on a reference coordinate axis, it may occur that they do not spatially match on the band and this means that there is no longer perfect continuity in the cut made in the band.
This discontinuity may be due to the fact that the start-up and stop operations of the band are not perfectly coordinated with the rotation of the die-carrier. In the field of application previous described there are other factors which also contribute to the existence of this discontinuity.
For example, the elastic nature of the material which forms the band can make it very difficult to coordinate these stop and start-up operations because the material may be stretched and delocalize the exact cutting start and end point. At the same time, an imperfect alignment of the cutting die during the installation thereof on the die carrier may also contribute to there being an interruption in the cut made, in this case because it may cause there to be a lateral discrepancy between a cutting end point and a cutting start point which should spatially match with the cutting end point. Other factors such as the maladjustment of the components of the cutting machine or the wearing of the cutting edges may also over time contribute to there being undesired interruptions in the cutting of a band.
These problems are not serious drawbacks when the band material is, for example paper or cardboard or when the thickness of the band largely exceeds the discrepancy between the cutting end and cutting start points. However, these problems of interrupting cutting are more significant when work is carried out with a material in the form of a film or thin sheet, like in the case previously described in terms of the net bags, since they can be the cause for subsequently manufacturing defective bags or for improvable quality and in particular circumstances tearing of the film and causing a stoppage to machinery.
Although today, the appearance of these manufacturing defects was assumed, a method and a cutting die, which solve these drawbacks, is an objective of the present invention.
A solution, which is applicable to flat dies and not only to rotary cutting dies, is also an object of the invention.